Sensors supply information about measurable physical or chemical (mostly non-electrical) quantities to control units in automobile applications, in industrial installation controls or in medical technology applications. Measuring transducers convert the measured quantities into electrical quantities (voltage, current, resistance, capacitance, frequency, etc.). As a rule, the conversion is subject to relatively large manufacturing tolerances of the measuring transducer and non-ideal situations such as, for example, temperature responses. In addition, the electrical quantities are typically very small and therefore not suitable for transmitting directly to a control unit. That is why, mounted at the measuring transducer is a signal evaluation circuit which conditions the electrical signal and routes it via an analog or digital interface to the control unit. This signal evaluation circuit is able to correct, for example, sensitivity, offset and temperature response in a part-specific manner. The data necessary for this purpose are stored in a non-volatile memory.
Such sensor evaluation circuits are increasingly designed as application-specific integrated circuits (so-called ASICs). Purely analog evaluation circuits with D/A (digital/analog) converters for the coefficients are customary. Digital evaluation circuits are also being used with increasing frequency, in which the electrical signal (with or without analog pre-corrections, for example, of the offset) is A/D (analog/digital)-converted and then subjected to digital signal corrections. Since the typical limiting frequencies of the measured quantities often lie below one kilohertz, but high demands are placed on the resolution of the sensors, the use of a so-called delta-sigma converter (delta-sigma modulator or sigma-delta modulator) presents itself for the A/D conversion of the electrical equivalent parameters which play a key role in such a system. This holds true in particular for modern IC processes, in which the component :density of digital circuits and the achievable switching speeds are increasing, while the analog qualities of the components are more likely decreasing. An example for a digital sensor evaluation circuit based on delta-sigma converters is known, for example, from the German Patent 100 34 813.
Furthermore, when working with measuring transducers having non-linear transfer characteristics and/or temperature responses, non-linear corrections in the evaluation circuit may also become necessary. Here, characteristics-map adjustments offer the greatest degree, of freedom. This is elucidated using a two-dimensional characteristics map shown in FIG. 2 as example: There, a physical measured quantity y_1 is dependent on two signals x_1 and x_2 by way of a non-linear function y_1=f(x_1, x_2). For example, x_1 could be an output signal of a measuring bridge and x_2 could be a temperature signal. The grid points of the curved plane, described by function f(x_1, x_2) in FIG. 2, may be stored in a characteristics-map memory. For each concrete measured-value pair (x_1, x_2), which generally lies between these grid points, the evaluation circuit must then undertake an interpolation with the four surrounding grid points as interpolation points, in order to ascertain an approximatively correct output value. The denser the interpolation points and the smaller the curve of the function in the respective direction, the more precise the interpolation becomes. For this reason, in FIG. 2, for example, the interpolation point density in direction x_2 is selected to be less than in direction x_1. In principle, the dimension of the characteristics map, thus, the number of input quantities x_i, is arbitrary, but often the cases occur one-dimensionally (so-called characteristic curve) and two-dimensionally.
The interpolation is accomplished with the four surrounding grid points of the characteristics map, often by the use of arithmetic-logic units. Thus, a microprocessor having suitable software, a digital signal processor (so-called DSP) or a special RISC (reduced instruction set computing)-processor may be used. However, often the costs associated with the implementation of such processor design approaches are not acceptable, particularly when using processes that employ less densely packed ICs, which, for instance, are used for applications in motor vehicles because of demands on the dielectric strength and reliability.
Another widely prevalent approach for non-linear corrections in the evaluation circuit is derived directly from a customary linear adjustment. In the linear case, signals are multiplied by coefficients, established in the adjustment procedure, and summed. For a non-linear adjustment, the coefficients may also come from a characteristics-map memory which is addressed as a function of signal and/or temperature. However, when working with a finite number of interpolation points, this leads to (mostly unwanted) sudden changes in the output signal when the addressing signal passes the interpolation points. Here, the use of oversampling methods may provide a remedy. In B. J. Hosticka: “CMOS Sensor Systems”, Sensors and Actuators A66 (1998), pp. 335-341, particularly p. 340, an oversampled temperature signal is used for addressing a characteristics map, which supplies coefficients for an analog signal-evaluation channel. The bandwidth of this analog channel is so low that a large part of the quantization noise of the interpolation-point quantization is filtered out again.